Applications to Chemical Reaction Rates

The effects of solvents on reaction rates have been studied most extensively on unimolecular solvolysis reactions (S l) and on bimolecular nucleophilic substitution reactions (Sj 2). Absolute rate theory specifies that the activated complex in the transition state is at equilibrium with the reactants and is formed on provision of the activation (Gibbs) energy, AG. In other words, the energy barrier that the reaction must pass to proceed has to be overcome. The specific rate constant is given by  

Empirical linear solvation energy relationships have, however, been widely employed, relating reaction rates to one or more properties of the solvents, hi common reactions involving ions, namely bimolecular nucleophilic substitution reactions (8, 2), it was shown that hydrogen bond-donating protic solvents increase 

Inorganic reactions exhibit their own trends, examples being outer-sphere electron transfer reactions of transition metal complexes reported by Wherland [70], In this case the rate depends on the difference rr -e because the high frequency solvent response, where n is the refractive index, represents the rapid response of the solvent to changes in the electric field produced by the electron transfer, whereas the permittivity responds much more slowly. Results for the electron exchange constants for ferrocene(0)/(l) studied by McManis et al. [72] and chromium bisdiphenyl(0)/(l) studied by Li and Brubaker [73] are shown in Table 8.7. The rate constants follow approximately the reverse order of the n - e function of the solvents. 

First-order solvent exchange rates at hydrated metal cations have already been described in Section 4.5.2 and listed in Table 5.4. Some similar data exist also for nonaqueous solvents methanol, ethanol, DMF, DMSO, and MeCN for di- and triva-lent metal cations. The rate constants at 25 C and, where available, also the enthalpies and entropies of activation have been reported in the book by Burgess [74]. The rate 

TABLE 8.6 Relative Rates log(fc mcoh) Nucleophilic Reactions (8.22) and (8.23) at 25°C [71] Compared with the Normalized Polarity index Ej 

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Kramers treatment of the escajje of a Brownian particle out of a potential welH" as a model for chemical reactions in condensed phases has played a central role in many areas of physics and chemistry. The original application to chemical reaction rates has in fact been disregarded by chemists until the last decade. Other applications were mostly in solid-state physics desorption... 

The general procedure described in this section was initiated by Kubo, Yokota, and Nakajima. The application to chemical reaction rates has been discussed by Yamamoto. ... 

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